Office of Management and Budget Director Mick Mulvaney, who has criticized the Congressional Budget Office, listens as President Trump speaks on March 17 at the White House. (Jonathan Ernst/Reuters)

The former directors of the Congressional Budget Office (CBO) — a bipartisan group that includes some of the nation's most eminent economists — published a letter defending the agency Friday morning, after weeks of strident criticism from the Trump administration.

President Trump's deputies have attacked the CBO publicly and relentlessly in response to its unforgiving analyses of Republican proposals to repeal parts of the Affordable Care Act, also known as Obamacare. Although the agency's current director, Keith Hall, was appointed by a Republican Congress, the administration has argued that the methods the agency uses are unsound and that its staff favors the Democratic agenda.

Partisan criticism of the CBO is nearly as old as the agency itself, which began operating in 1975. All the same, those criticisms have been particularly intense this year, amid a political environment in which partisan disputes over basic facts and arithmetic have replaced debates over values, principles and visions for the place of American government in society.

The unusual letter reflects that environment, although the text does not mention any specific criticisms or the White House explicitly. It is addressed to the top Democrats and Republicans in the House and Senate: House Speaker Paul D. Ryan (R-Wis.), House Minority Leader Nancy Pelosi (D-Calif.), Senate Majority Leader Mitch McConnell (R-Ky.) and Senate Minority Leader Charles E. Schumer (D-N.Y.).

“Relying on CBO’s estimates in the legislative process has served the Congress — and the American people — very well during the past four decades,” the authors write. The signatories of the letter include Douglas Holtz-Eakin, a conservative supporter of the GOP effort to dismantle Obamacare, as well as Peter Orszag, who served as budget director in the Obama administration.

The CBO is tasked with providing lawmakers with impartial information about the federal budget, taxes and the national debt.

According to the latest projections from the agency, issued Thursday, the Senate's version of the GOP bill to undo Obamacare would result in some 22 million additional Americans going without insurance after a decade.

The federal government would save money, and premiums would decrease overall in the individual market under the GOP plan, the CBO said. But at the same time, some consumers — particularly older adults in the upper middle class — could pay thousands more in premiums, and typical deductibles could increase to as much as $13,000 a year.

The Trump administration has argued that the CBO failed to forecast the effects of Obamacare accurately when it was initially passed. The agency overestimated the number of people who would gain insurance under the law. In particular, according to the administration, the CBO places too much emphasis on Obamacare's requirement that every American carry health insurance, which in practice has been weakly enforced and widely ignored.

“Unfortunately, even nonpartisan and high-quality analysis cannot always generate accurate estimates,” Friday's letter reads. “Policy changes are often complex, the economy is dynamic and defies precise prediction, and many policies are modified over time.”

Yet while those criticisms may be legitimate, the White House has gone further, with personal attacks on the agency's motives and credibility, said Craig Garthwaite, a conservative economist at Northwestern University. The publicity campaign has included a video on social media, and Trump's budget director, Mick Mulvaney, has described CBO's career analysts as Democratic sympathizers.

The administration's own claims about the effort to replace Obamacare with a new system have been suspect. Tom Price, Trump's secretary of health and human services, said last weekend that the GOP plan would cover more people — a claim that few economists find persuasive.

“We should be looking to make good health policy,” Garthwaite said. “We shouldn’t be looking to make ideological health policy and then try to dress it up in fancy numbers.”

Congress established the CBO in part because legislators needed an independent source of analysis on the federal budget — apart from what the executive branch was telling lawmakers. That mission has always resulted in tension between the CBO and the president.

In 1981, an unfavorable report on President Ronald Reagan's budget from the CBO resulted in a failed effort by his allies to oust the agency's director at the time, Alice Rivlin. Rivlin signed the letter along with the other seven full directors of the agency.